With an increasing power density, thermal management of power electronic devices, such as for example, variable speed drives, can become an issue. For many applications, for example in the food and beverage industry, a high ingress protection class (IP class) can be required. In these cases, the electronic components can be enclosed in a sealed housing, which constrains the cooling techniques applied.
Heat losses can be generated in almost any electronic component. In the case of variable speed drives, a major source of losses can be the power module, i.e. the power semiconductor switches. A commercial low-voltage drive with a nominal output power of 4 kW can have heat losses distributed over the individual components, as shown in Table 1. It is to be noted that the values given in Table 1 are examples and do not refer to any specific device. Desirable components for cooling are the power module, due to the high power density, and the electrolytic capacitors of the DC link and auxiliary power supply, due to their temperature sensitivity. As seen from Table 1, the power module can be a high-loss high-temperature component, whereas the other listed components can generate considerably lower losses.
Electronic components can be cooled with air by forced convection at ambient temperature. However, this approach may not be possible for an electronic device enclosed in a sealed housing.
TABLE 1Distribution of heat losses in a commercial variablespeed drive with 4 kW nominal output power.ComponentMax. temp.(° C.)Heat loss (W)Power module12570 W DC capacitors8515 W Balancing resistors1252 WControl electronics1255 WAuxiliary power supply858 WFan705 WSum105 W 
An electronic device that can have a high ingress protection class can be cooled by natural convection, as shown in FIG. 1. FIG. 1 shows a structure in which natural convection can be enhanced by shaping a part of the enclosure as an extended surface. In the case of variable speed drives, the power module, having the highest heat losses, can be directly attached to a heat sink, which can minimize its junction to ambient thermal resistance. The low-loss components can be arranged on circuit boards 11 located inside the sealed space. For cooling, an internal fan 12 circulates the air inside.
One thermal bottleneck can be the heat rejection from the internal air through the enclosure to the ambient air. This bottleneck can cause an increased air temperature inside, which yields higher component temperatures and therefore can decrease their lifetime. Due to the power module's heat losses, the temperature of the baseplate of the heat sink is usually relatively high, which can cause an additional heating of the internal air.